Valve Stem Seals S type 4.2 V8 2003

My engine has done over 350000km and would clearly benefit from a full rebuild, but that is something I can’t afford. I’m prepared to drive it accordingly and it runs very well except for clouds of smoke if I take off too hard when I’ve sat for a time at lights or waiting to make a turn. I love the shim over bucket valve clearance adjustment as even though I haven’t done mine yet the adjustment can be made without removing the camshafts. However replacing the valve stem seals is made more complicated with the heads on by this setup. Has anyone replaced their seals and can offer any advice? Also, if I decide to go ahead, does anyone know if there has been an upgrade from the original seals or can recommend the best brand to use? On the other hand given the high mileage do you think I’d be wasting my time?